In the past decade a new direction in the field of ion-selective electrodes has emerged with the development of potentiometric sensors with plasticized polymeric membranes for the detection of polyionic macromolecules. Early work in this area proposed a polymer membrane electrode containing a lipophilic anion-exchanger, which was capable of detecting the polyanion heparin. See Ma, S. C., Yang, V. C., and Meyerhoff, M. E. Anal. Chem. 1992, 64, 694. Heparin-selective polymeric membrane electrodes are further described in U.S. Pat. Nos. 5,236,570 and 5,453,171.
Heparin is a highly sulfated polysaccharide with an average charge of −70 and an average molecular weight of 15,000 Daltons. The molecular formula for one unit of a heparin compound is provided below.

Heparin is used as an anticoagulant in major surgical and extracorporeal procedures, such as open-heart surgery, bypass surgery, and dialysis. The use of excess heparin in medical procedures can be detrimental, however, necessitating precise monitoring of heparin administration. Real-time monitoring of heparin concentration in blood is particularly useful for preventing the risk of excessive bleeding during operations and reducing postoperative complications. Activated clotting time measurement (ACT) is a common method for estimating the heparin concentration in whole blood. Although this method is widely used, it is nonspecific and indirect, and the results can be affected by many variables. In contrast to ACT, the heparin-selective electrode is able to detect heparin concentration directly in whole blood or plasma samples.
Similarly, an electrode for sensing the polycation protamine has also been proposed. See Yun, J. H., Meyerhoff, M. E., and Yang, V. C. Anal Biochem. 1995, 224, 212. The polypeptide protamine is generally used for neutralization of heparin activity (i.e. to promote coagulation). Protamine, which is illustrated below, is a polycation with an average charge +20 and is rich in arginine residues.

The basic guanidinium groups of protamine complex electrostatically with the sulfonate groups of heparin to render the anticoagulant activity of the heparin ineffective. Excess use of protamine, however, can also be detrimental. For example, the use of protamine frequently results in adverse hemodynamic and hematologic side effects, such as hypertension, depressed oxygen consumption, thrombocytopenia with pulmonary platelet sequestration, and leukopenia. It is therefore useful to be able to accurately detect and measure protamine concentration in biological fluid, such as blood.
Reliable detection of protamine allows for careful administration of the agent, thereby avoiding the associated problems noted above. Further, with the ability to detect protamine via ion-selective electrodes, it is also possible to determine the heparin concentration in a sample via titration of the sample with protamine. This is possible due to the specific heparin-protamine interactions described above. Such action is also described by Ramamurthy, et al., Clin. Chem. 1998, 606.
The observed response of the heparin-specific membrane electrode known in the art could not be explained in terms of classic equilibrium approach. The Nernst equation should yield a slope of the electrode function of less than 1 mV/decade and 2 mV/decade for heparin and protamine respectively, because of the high charge of these ions. A quasi-steady-state model to explain this unusual mechanism was subsequently described. See Fu, B. et al., Anal Chem. 1994, 66, 2250. The potentiometric polyion sensor response is kinetic in nature. A strong flux of polyions occurs both in the aqueous solution and the membrane phase due to the spontaneous extraction of polyions into the polymeric membrane and the concomitant exchange with hydrophilic ions from the membrane, which results in a potential change in the presence of polyions.
Because the extraction of polyions is an irreversible process when using the heparin-specific membrane electrode of the prior art, a strong potential drift is normally observed. After a relatively short time in contact with a polyion solution the sensor starts to lose its response. Extracted polyions must be removed from the membrane phase by reconditioning of the sensor, such as in concentrated sodium chloride solution. Multiple methods have been proposed in the art for overcoming response loss due to polyion concentration at the membrane surface. A pH cross-sensitive potentiometric heparin sensor has been proposed, wherein the sensor contains an ion-exchanger and a charged H+ ionophore. According to this method, heparin stripping could be accomplished by adjusting the pH of the sample. Another approach for overcoming lost sensor response is to use disposable sensors.
Thus, despite the existence of a selective extraction principle, it has been impossible thus far to design a reversible polyion sensor. Accordingly, while polyion sensors can be highly useful in critical care applications, their use is limited by the quick loss of response of the sensor. Single use sensors lead to increased expense, and the necessity of removing the sensor and reconditioning the sensor by a separate methods is overly time consuming and adversely limiting on the usefulness of the sensor. Therefore, it would be useful to have a sensor for detecting polyions that is fully reversible, wherein such reversal can be performed quickly, repeatedly, and without removing the sensor to a separate solution.